(1) Field of the Invention
The present invention relates to a hot-melt adhesive composition and, particularly, to a hot-melt adhesive composition having excellent heat-resistant adhesiveness and heat-resistant creeping property.
(2) Description of the Prior Art
Attempts have been made to use low-molecular polymers and petroleum resins such as styrene, vinyltoluene, .alpha.-methylstyrene and the like as stickiness-imparting resins that are to be blended into rubber-type pressure-sensitive adhesive agents and to ethylene/vinyl acetate copolymer-type and styrene block copolymer-type hot-melt adhesive agents. However, when the above resins are used for the rubber-type pressure-sensitive adhesive agents, a sufficiently large adhesive force or sticking force is not obtained. When the above resins are used for the hot-melt adhesives, furthermore, compatibility becomes poor with respect to the ethylene/vinyl acetate copolymer which is a chief component of the adhesive agent. On the other hand, petroleum resins produced from the ordinary fractions of distillates containing unsaturated hydrocarbons involve problems in regard to hue and thermal stability.
In order to solve these problems, the present inventors have previously proposed the use of a polymer which is obtained by polymerizing at a particular ratio the isopropenyltoluene and any fraction of distillate selected from those fractions of distillate having 4 to 5 carbon atoms that are by-produced at the time of refining petroleum or cracking petroleum, as well as the use of a homopolymer of the isopropenyltoluene (Japanese Laid-Open Patent Publications Nos. 118729/1974 and 128945/1974 and Japanese Patent Publication No. 34033/1979).
Though the above-mentioned problems were improved by these proposals, however, lack of heat-resistant adhesiveness and lack of heat-resistant creeping property in the hot-melt adhesive agent have not still been solved. It has further been known that the above-mentioned homopolymer of isopropenyltoluene helps improve the heat resistance when it is used for the hot-melt adhesive agent of the styrene block copolymer type, but makes it difficult to obtain high softening-point products having softening points of higher than 120.degree. C. maintaining good yields. Furthermore, a resin that has been placed in the market as a resin to impart heat resistance, has a high: molecular weight and helps improve heat-resistant creeping property. This resin, however, exhibits poor compatibility with respect to the base resin that is a chief component of the adhesive agent, and gives such defects as decreased tacking property of the adhesive agent and increased melt viscosity of the hot-melt adhesive agent making the operation of application difficult.
As a stickiness-imparting agent for improving the hue and the heat resistance, furthermore, there has been proposed a nuclear-hydrogenated copolymer resin of a cyclopentadiene, a dicyclopentadiene or an alkyl-substituted product thereof and an indene or an alkyl-substituted product thereof (Japanese Patent Publication No. 8849/1986). After the polymerization, however, a step is necessary for effecting the hydrogenation, causing the process to become complex and the cost to become high.